Denoising not working with GPU
I did a little testing on the new egpu I have for my imac and there is definitely an incompatibility on my system between the board and the denoiser.

The incompatibility seems to make everything black with the Glossy Direct turned on. 

With Glossy Direct turned off, everything is wicked contrasty after denoising.

With the glossy indirect turned off, everything is extremely dark after Denoising.

Playing with the different denoising levels doesn't change any of that.

I am running an AMD Radeon R9 in an Akitio enclosure, with a 2015 imac. Renders are faster but I definitely wish I could run the denoise to cut down on samples.

Hopefully, the incompatibility will get fixed in the next version.